1. Field of the Invention
The invention relates to an electret transducer comprising a diaphragm plate and a backplate with spacing means between the diaphragm plate and the backplate for providing multiple supports between the diaphragm and backplate, and a method for manufacturing a backplate, electret, and diaphragm plate assembly.
2. Prior Art
An electret transducer of the above kind is known from U.S. Pat. No. 4,160,881 issued July 10, 1979 and entitled "Improved motor assembly for an electret transducer and a method for manufacturing such a motor assembly."
According to this prior art the multiple support between the diaphragm plate and the backplate is provided by means of a meshlike screen disposed on the electret foil covering the upper backplate surface. Around its circumference such screen is fixedly attached to a mounting rim supporting the backplate and diaphragm plate.
Although in principle the operational characteristics such as for instance transducing efficiency of such transducer can be improved and controlled with such a prior art transducer structure, in practice the yield obtained when such transducers are manufactured in a mass production process is less than could be expected. Moreover the sensitivity of such transducers is deleteriously reduced due to the fact that spacing screen is fixedly attached to the supporting mounting rim. Unsurmountable manufacturing problems arise when it is attempted to improve the sensitivity by specific selection of design parameters. Moreover this approach fails to lead to a mass production process whereby a high yield is ensured for electret transducers which should meet stringent specifications.
U.S. Pat. No. 3,740,496, issued June 19, 1973 and entitled "A diaphragm assembly for an electret transducer", also discloses an electret transducer of the above kind. According to this prior art multiple support between the diaphragm plate and the backplate is provided by means of diaphragm supporting members which are formed integrally with the backplate. More in particular the supporting members are upwardly extending indentations formed in the backplate and are therefore formed from the backplate itself. This prior art structure is liable to the same deficiencies as has been outlined above. Moreover this prior art structure is unappropriate for manufacturing transducers having accurately reproduceable characteristics such as sensitivity and frequency response.
U.S. Pat. No. 4,070,741 teaches a method of making an electret transducer, wherein posts for supporting the condenser transducer diaphragm in relation to a backplate are formed on the diaphragm film itself by selectively etching away a photoresist material laminated on the film. The electret material is then supported on the backplate rather than carried by the diaphragm. Apart from being relatively complicated and uneconomical, this prior art method leads to a relatively high parasitic capacitance thereby degrading the transducer performance; moreover it requires the height of the backing plate of being adjusted with respect to diaphragm mounting rings so that the posts lift and slightly distort resp. the diagram. This prior art is inappropriate for providing a mass production process ensuring a high yield of electret transducers which should meet stringent requirements within close tolerances.